In a medium pan, turn the heat to medium. Add butter and cut onion. Cook until they get soft.
Move the cooked onion to a medium bowl, and cool the onion down.
After the onion is cooled down, add minced shrimp, potato starch (or Katakuriko), salt, and black pepper. Mix everything.
Divide the mixture into 4 servings, and shape each of them into a flat circle.
Add flour around each patty. Dust with extra flour.
Then, add it to egg bowl.
Then, add panko for each patty.
In a pot, add oil and bring to the heat to 355°F (180°C).
Add two patties into the pot at a time. Deep fry each side for 1 minute. (Total 2 minutes)